This update allows us to access volumetric path tracing
and other new features of OSPRay 2 in VTK.

However, since OSPRay2's raycast or "scivis" renderer is
simpler than before, we lose shadows and multiple
and colored light sources, gradient shaded volumes and
specular highlights in that context. The regression tests
and baselines have been adjusted accordingly.

This commit compresses a long development branch by
Carson Brownlee and David DeMarle. A selection of the commit
message are as follows:

RayTracedMolecules will be restored in the next commit

-
    style fixes for ospInit and ospShutdown call sites

    Returning ospShutdown resulted in segfaults in OpenVKL. So let's
    leave it as always on once started. VisRTX does the same and
    we've done it this way in earlier releases so it will probably
    be fine.
-
    restore temporal caching

    New approach is to simply cache the entire scene instead of
    individual props.
-
    Update the ospray isosurface baseline image.

    Comparing with GL shows that the new version is more similar to
    GL so it is valid.
-
    compensate for lack of gradient shading in ospray2 vol rendering

    until ospray 2 gets diffuse volume shading, turn off shading and
    update baselines.
-
    compensate for raycaster lighting limitations

    until ospray2 raycaster gets colored lights, switch these two
    tests over to pathtracer to make them happy.

    Also add a new alternate image for the lights test because
    path tracer is much brighter in corner than ray caster is.
-
    update to ospray 2's revised material names

    To save a bit on runtime cost, we only provide backward
    compatibility at material library file parsing. In other words, user
    code that calls MakeMaterial() directly may have be updated.
-
    changing ospray version number to 2.1
-
    restructuring renderer to use list of instances instead of models.
-
    adding isosurface material
-
    using 3d data array for volume data
-
    adding error logging.
-
    adding volume anisotropy
-
    intial commit of ospray 2.0 rendering code base.